Cellulose is widely used as a raw material for the production of articles such as rayon and cellophane. Most commercial methods for processing cellulose involve exposing the raw cellulose to an aqueous alkali solution, then adding complexing agents to produce a dope. The dope is then subjected to various processing steps to regenerate cellulose. The cellulose produced by these methods is known as regenerated cellulose because the cellulose molecule is dissolved by the production of specific coordination complexes with the dissolving solvent. For example, according to one method the raw cellulose is first steeped in sodium hydroxide. Carbon disulfide is added to the resulting alkali cellulose crumb under specific conditions, resulting in the formation of sodium cellulose xanthanate. The coordination complex is then converted into the regenerated cellulose. In this process, the intramolecular hydrogen bonding in the cellulose is disrupted through the formation of coordination complexes between the alkali and the glucose of the cellulose.
Two common methods for the production of regenerated cellulose are the viscose method and the cuprammonium method. Each of these methods utilizes a step-wise chemical process wherein cellulose is dissolved into solution by converting it into a cellulose derivative. The resulting dope containing the cellulose derivative is then processed into regenerated cellulose derivative products by, for example, wet spinning, extrusion and coagulation.
These methods have been successfully used for the production of regenerated cellulose on a commercial scale for many years. However, each method utilizes expensive materials, and each produces certain toxic by-products. Thus, there exist environmental and economic concerns inherent with each method. In addition, because the glucose moieties of regenerated cellulose produced by either the viscose method or the cuprammonium method exists as a coordination complex, the degree of polymerization of the molecule is reduced. In other words, as the number of coordination complexes in the regenerated cellulose increases, the intramolecular hydrogen bond forming sites on the cellulose molecules decrease. This generally decreases the commercial desirability of the resulting product because an increased number of intramolecular hydrogen bonding sites generally produces higher quality finished products.
Other methods for dissolving cellulose for the production of shaped articles are well known. One such method is through the use of organic solvents such as N.sub.2 O.sub.4 /DMF (dinitrogen tetroxide/dimethylforamide), DMSO/PF (dimethyl sulfoxide/paraformaldehyde), and TFA/CA (trifluoroacetic acid/chlorinated alkanes). Methods utilizing these organic solvents are generally not used on a commercial basis because of their complexity and prohibitive cost. In addition, these methods use toxic components which pose environmental hazards.
Various metal complexes also may be utilized to dissolve cellulose. For example, LiCl/DMA (lithium chloride/dimethylacetimide), Cadoxene (cadmium/ethylene diamine), Coxene (cobalt/ethylene diamine), and CUEN (cupriethylenediamine) have been used. Like organic solvents, these compounds are generally not used on a commercial basis for the production of regenerated cellulose because the chemical components are expensive and contain toxic materials, such as heavy metals and amines.
Shaped products such as rayon and cellophane may also be produced without dissolving the cellulose into solution as a coordination complex. One process for dissolving cellulose without production of an intermediate coordination complex is disclosed in U.S. Pat. No. 4,634,470 to Kamide et al. Kamide teaches a method whereby the raw cellulose molecules are cleaved by disruption of the intramolecular hydrogen bonding, but without the intermediate step of forming coordination complexes. In one approach described in Kamide, a mixture of 100 parts by weight of cellulose and 10 to 1000 parts by weight of hydrogen cleaving solution, such as aqueous solutions of alkali or organic acids, is held at a temperature between 100.degree. C. to 350.degree. C., under a pressure between 10 to 250 atmospheres. The pressurized mixture is abruptly discharged into ambient atmospheric pressure, which causes a rapid volatilization of the hydrogen bond cleaving agent. This volatilization in turn disrupts the intramolecular hydrogen bonds, causing dissociation of the cellulose molecule. The resulting fibers from this steam explosion process are fragmented. That is, the abrupt discharge results in the shortening of the cellulose structure which will result in a lower degree of polymerization in the final product. The resulting shortened fibers are then mixed with an alkali solution to dissolve the cellulose to produce a dope which is described as having a degree of polymerization of at least 100. The steam discharge is understood to have limitations on the maximum degree of polymerization attainable. The article Characterization of Cellulose Treated by the Steam Explosion Method, Brit. Polymer J. 22 (1990) by Yamashiki, et al., suggests that in the steam explosion process, the degree of polymerization of the initial cellulose fibers had to be reduced substantially in order to obtain satisfactory dissolution of cellulose in a cellulose dope.
Unlike the regenerated cellulose produced by either the viscose method or the cuprammonium method, which as noted utilize cellulose derivatives, the cellulose dope produced according to the steam explosion method described by Kamide requires neither intramolecular hydrogen bonding in the dissociated state nor coordination complexes. As a result, the shaped articles produced from the dope reportedly have improved mechanical properties and chemical resistance. However, as with any high pressure process, safety concerns are present with the Kamide, et al. approach. The Kamide, et al. patent also describes processes involving the regeneration of cellulose produced using, for example, a solvent such as lithium chloride. The regenerated cellulose is then dissolved in an aqueous alkali solution (e.g. NaOH) having a concentration of 6-12 percent by weight at a temperature not higher than 50.degree. C. preferably less than 10.degree. C., and most preferably less than 7.degree. C. This process suffers from the disadvantages previously mentioned in connection with approaches involving the production of a regenerated cellulose.
It is also well known in the art that cellulose swells when exposed to the hydrogen bond cleaving agent sodium hydroxide. Swelling of the cellulose by nearly 1000 percent is achieved upon exposure to sodium hydroxide maintained at soda cellulose Q condition. This is explained in an article by Sobue, et al., entitled "The Cellulose-Sodium Hydroxide-Water System as a Function of Temperature", Z. Physik. Chem. (B)43(3), 1939. Soda cellulose Q condition occurs when cellulose is in an aqueous solution of 6 percent to 10 percent by weight of sodium hydroxide, at a temperature between -7.degree. C. and 4.degree. C. The aqueous sodium hydroxide penetrates between the fiber layers. This expands the lattice structure and results in the formation of large, irregular distances therebetween. However, only the amorphous fraction of Cellulose I fibers (e.g. about 20-30 percent) dissolves under these conditions.
Since the late thirties, sonics have been used in the papermaking industry to defiber and fibrillize pulp fiber to achieve a fiber of high quality and strength for use in making paper products. More recently, sonics have been used to disperse high gloss inks and overprint varnishes from recycled paper. After the recycled paper is de-inked, the pulp from the paper is reused in the production of other paper products. However, despite the long-standing availability of this technology, no one has applied this technology to the production of cellulose dope.
Therefore, a need exists for an improved method of producing a cellulose dope.